My friend cannot connect to this server.

So last night a friend of mine who hasn't played TF2 in a while started playing again. Long story short he kept getting "server not responding errors" when trying to connect. We tried various solutions but nothing worked. He was able to connect to 2fort and other non-iO servers just fine. Gold/bad had about 20 something people on it at the time and I was playing on it without issue when he was trying to join.